Hmm... I have several "Live" distros lying around... one absolute minimalist on a single floppy for simpler things like disk formatting diagnostics etc, Feather Linux on my USB pen and on CD, and Knoppix (CD). Basically I ended up with a whole bunch while looking at different distros. I also have install discs for Ubuntu and Slackware (disk 1 only) and am currently trying to install Slackware. Basically I've ended up with so many because I was looking for a good distro for my old box, and I have only CD-Rs, no RWs.
For live distros I'd say:
Knoppix if you want a bigger CD/DVD distro (several gigs of software compressed on each, and obviously there's more on the DVD version) if you want to try a pretty full featured distro with lots of different apps (office, art/design, games etc etc) and several different options for different desktop environment (Default is the very nice looking KDE, but there are also "lighter" ones like Fluxbox, IceWM and TWM)
Feather Linux if you want a nice light distro that also packs a lot of apps in. It's actually a remastered version of Knoppix, and uses a lighter desktop environment and small apps. Total size is <128Mb, and you can run off a USB pen drive or CD. Pen drive is useful because you can save settings and your work on the same portable medium as the OS itself, instead of carrying around a CD and rewritable media. DSL (Damn Small Linux) is also apparently very good for this sort of thing, and is much smaller, but consequently also has less software.
Hope that helps
Good luck, and hope you enjoy it.
EDIT: For a fairly comprehensive (or at least BIG) list of Live distros, try www.linux.org, a pretty good Linux-centric site. In the sidebar click "distributions", then on that page, select "Live CD" from the drop down list titled "category" and click GO. In fact that's fairly useful for finding whatever type of distro you want